3/4 to full fouling plugs

After solving my electrcal problem my jetting is off and I have been fouling plugs when riding trails and getting on and off the throttle and running WOT. If I ride 3/4 and below the plug takes alot longer to foul. My question is do you think it's the main. I just put a leaner needle OBERT and diafram 3.3 in and was hoping this would solve the problem, but didn't. I have a 152 main and was thinking of dropping it in, but I am looking for a second opinion.

Come on now I'm looking for a second opinion, and I know quite a few of you usually have one.


In my experience if you are rich on the main there will be alot of missing at WOT.

OK, I'll throw out a couple of thoughts for ya.

1) Valve adjustment. Recent?

2) Exhaust. Clogged spark arrestor.

3) Spark Plug. Hotter vs what you have.

4) Air box. Add some holes (with screening)

5) Air Filter. Too much oil restricting air flow

I'm sure you have eliminated some of these already, but you wanted comments so .... :)

Thanks folks, being a good bicycle mechanic don’t help much after you slap a motor into the mix. This bike has been driving me crazy lately. My spark plug cap was shorting out and causing problems. I thought it was jetting messed with everything, figured it out, went back to the old (good) jetting. The bike ran great for about two weeks, and then started fouling plugs again.

I have been wondering if it might be the valves, they haven’t been checked for about 8 months. How quickly can they get out of tolerance, and what are some of the symptoms of valves out of spec. Thanks for any info.


The valves seat over time. Once they get to a certain point they will require more frequent adjustment and finally must be replaced. When valves start seating they don't open as much as they should. For example, this would result in the exhaust valves not opening sufficiently for spent fuel to be efficiently pushed out. This will result in a fouled plug. Overly tight valves will cause severe engine damage.

On the other hand, if the valves are too loose they don't close completely. During the combustion stroke loose valves become very hot around the edges and can develop burn spots. Loose valves result in a loss of power, first indication is lack of a snappy response.

If the bike is running well, no chatter is evident and you are an occassional nonracing rider, then 8 months can pass without checking. I usually check mine at 500 mile intervals. However, in your case I would check them now.

I qualify my commnets with my "Shade Tree Mechanic" certification. :)


What gas are you burning. There are many reasons a plug will foul. Gas being one, prayer mentioned another the plug make sure the plug is the correct one. I dont recommend going hotter for now, find the issue first, going hotter with out identifying the issue may just burn a hole on the piston. Valve adjustment is another but not an leading indicator.

Here is what I would look for

1: Blue Wire Mod (Do A search on Blue Wire or go to Motomans site ar http://motoman393.thumpertalk.com look under articles and blue wire.

2: GB/BKmod look under motomans site again for this and do a search lots of information

3: Gas believe it or not this is one of the big reasons plugs have been noted to foul. Not sure what you use but use a high octane 91 or better try a name brand (Chevron, Union, Shell)

4: Carb -- Make sure the carb is spotless, take it off clean it and make sure the float height is in spec. if it is sticking or open this can allow gas to freely flow in a WOT condition.

5: Air filter Make sure its clean dont over oil and make sure you let the oil set up after applying.

I hope I covered most of it

Oh ya, You did mention you just went though electrical, a bad TPS can cause the fouling as well, did you replace that.

It is really random how the bike will run great and then like crap, it is almost like it hits a certain temp and then start a fouling. Put a new plug in and it runs like a champ until it fouls again. It is really strange how the bike is pulling wheeles in 4th and two minutes later it's loosing power (fouled) and then won't even run. New plug and no problems until it happens again. I am about ready to take it to a dealer, but I don't trust the local Yamaha shop (long Story).

To answer some of your questions.

Decent gas 91 octane, texaco/66 normally.

Grey but not blue mod.

Getting ready to BK.

Carb is spotless (I have been taking it off every week since Jan).

CR9E Spark plug

Two air filters so one is always clean.

Suspicious about all electrical after short in cap.

The only other thing I have noticed is some times there is a decent amount of drip? from breather hose.


Check out this article:


You said you have a bit of crankcase breather hose drip. How much are we talking about? How much oil are you having to add and how often?

I had the same problem. Was one of two things, but not sure which since I did both at the same time.

Found the inlet tube on the petcock had come off and gas was coming in through the open hole and not getting filtered. I kept finding crud in my carb. I put the tube back in, put an inline fuel filter on, and cleaned the carb again.

The other thing I did was cut about a half inch off the end of the spark wire and screwed the cap back on.

Haven't fouled a plug since. :)

Good info on plugs. I am running the CR9E, because the local mechanic said it would help up here at elevation. I will go out and get an 8E, pop it in see what happens.

I already cut the 1/4 inch off of the plug wire end, I will look at the petcock. Thanks for all the help.

the local mechanic said it would help

That will always get you into trouble :)

If changing the plug back to a CR8E doesn't work, dig into your jetting a little more. It sounds to me like you are way to rich on your main to be fouling a plug at 3/4 to WFO. Before I went to JD jetting, I ran a 38PJ/148MJ/stock MAJ/EKQ #2/75 PAS/1.5 turn out PS and it worked great from 8,000 to 13,000 feet. By running a 160MAJ (I think the stock MAJ is a 200, which would mean that you richened the main by dropping it to a 160) combined with a 155 MJ, you are way rich and needed to compensate by going with a leaner needle.

Did you recently change your exhaust? Is something cloging the air flow (filter, air jets)? Cleaning your carb can't hurt and it may be that something is pluging your air jets.

Just throwing some ideas out there for ya. :D

When I put the plug in I will drop in my 152 and see if that helps. I put a new pipe on last january, the bike ran great until spring when all of my problems started. So I thought it was just jetting ie.. hotter/richer. Turns out it was probably electrical and jetting. I love this bike but I have had alot of trouble with it the last 6 months, I just wish I wasn't such a shade tree mechanic :) but you gotta learn some how.

The CR8E plug is ok, But the CR7EK is the plug of plugs. 10 times more resistent to fouling and is hot enough to burn the fuel better. Dropping the main is a good idea. The CR7EK plug will NOT hurt your piston or cause your motor to over heat. I can send you photos of my recently replaced top end if you like.

Bonzai :)

Well I did a couple of things. First I dropped the main to a 152 and cleaned the carb. Secondly I checked into plugs. I went with the NKG CR8EK plug. I test-road it to work this morning and all seems well so far, but only a hard ride will really tell. Thanks to all for your info/help. :)

While talking with a different shop owner than the one who advised the CR9E, he said that he believed that NKG was the only plug manufacturer who 's numbers go from bigger (hotter) to smaller (Colder), or (CR10E=COLDER_CR6E= Hotter). Does anyone else know this?

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